A early 50s, 4250, the 4450 came out on the 500 FQ which was introduced in 55,There should be a screw slot in the end of the brush cap, you just unscrew the cap and pull out the brush.
The screw in the frame above the brushholder just holds the bakelite brushnolder in place. It's like Ken said there should be a brass plug in the bakelite that has a slot in it. You just unscrew that and the brush and spring will come out. Sometimes if the brush is burned you may have to take something and push the brush back through the holder. Be careful around the armature, you don't want to damage any of the segments.